Conventionally, a method has been used in which a punch is pressed from one end of a steel pipe to form an expanded pipe part.
For example, Japanese Patent Laid-Open Publication TOKUKAI No. 2002-346664 (published on Dec. 3, 2002) discloses a pipe expanding method for an end of a metal pipe that uses a punch with a taper angle in a range 15 to 30 degrees, and a metal pipe holder having a tapered portion with the same taper angle as the taper angle of the punch so that the punch is pressed from the end of the metal pipe to expand the end of the metal pipe.
According to this pipe expanding method, a pipe end of a metal pipe can be efficiently expanded that is formed of stainless steel or and high-strength steel with a high yield strength and a high work hardening coefficient without deformation such as buckling or flattened shape.